The present invention relates to the vacuum tube arts. It finds particular application in connection with reducing bearing assembly temperature in a rotating anode tube during operation as well as during the exhaust and baking process, thereby increasing bearing assembly life and rotating anode tube life and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable for reducing bearing assembly temperature and increasing bearing assembly life in other vacuum systems.
A high power x-ray tube typically includes a rotating anode disposed within a glass envelope. A cathode supplies an electron beam to a target surface of the anode. When a high voltage differential causes the electron beam to strike the rotating anode, the beam heats the surface of the anode while generating x-rays which pass out of the glass envelope.
An induction motor is typically provided for rotating the anode. The anode is configured to rotate so that the heat energy will be spread over a relatively large area, thereby inhibiting the target area from overheating. The induction motor includes driving coils positioned outside the glass envelope and a rotor within the envelope which is connected to the anode. The rotor includes an outer, cylindrical armature or sleeve and an inner solid bearing member, which is centrally aligned within the armature. The armature and bearing member are centrally connected to the anode by a neck. A cylindrical bearing shaft is axially aligned with the armature and bearing member and is positioned therebetween. The bearing shaft is connected, at a rearward end, to a housing disposed outside the envelope.
When the motor of a typical x-ray tube is energized, the driving coils induce magnetic fields in the armature which cause the armature and bearing member to rotate relative to the stationary bearing shaft. Bearings, such as ball or roller bearings, are positioned between the bearing member and bearing shaft for allowing the bearing member, armature, and anode to rotate smoothly, relative to the bearing shaft. The bearings are positioned between bearing grooves provided in the bearing member and bearing races provided on the stationary bearing shaft. The bearing grooves and bearing races help maintain the proper positioning of the ball bearings. The ball bearings are typically coated with a solid metal lubricant. A metal lubricant is typically used, rather than a standard petroleum based lubricating compound, because the x-ray tubes operate in a vacuum requiring low vapor pressure. During normal operation, cooling oil is circulated to cool the bearings, preferably below 350xc2x0 C.
During the manufacture of x-ray tubes, it is common to have the x-ray tubes undergo an exhaust process which removes unwanted molecules, such as water, from within the glass envelope. The exhaust process typically includes placing a number of x-ray tubes on individual stands in an oven to bake for a predetermined amount of time at a temperature of about 350xc2x0 C. While in the baking oven, the interior of the tubes is connected with a vacuum pump which removes unwanted gases and molecules. The gases and molecules are outgassed during the baking cycle. Electrical and cooling oil connections are not made in the oven.
Under the heat of the baking oven, the lubricants on the bearings become hot and tend to evaporate and redistribute. The evaporation and redistribution of lead lubricant from a bearing race accelerates rapidly at temperatures over about 350xc2x0 C. These temperatures can be reached in the bearings during the exhaust process. The evaporation and redistribution of the lead lubricant leads to a rapid degradation of the bearing surfaces and premature tube failure.
Thus, while it is advantageous to bake the x-ray tubes at higher temperatures for more efficient outgassing, the baking temperature is limited by the lubricant on the bearings. Previously, attempts to reduce the bearing temperature during the exhaust process included applying cool air to the exterior surface of the glass envelope. However, because the bearings are located deep inside the glass envelope, such a method does not provide much cooling assistance.
Moreover, the lubricants on the bearings also become hot and tend to evaporate and redistribute during normal operation of the x-ray tube. Again, evaporation and redistribution lead to undesirable degradation of the bearing surfaces and premature tube failure.
The present invention provides a new and improved bearing assembly and method of cooling for an x-ray tube which overcomes the above-referenced problems and others.
In accordance with a preferred method of preferentially reducing bearing temperature in an x-ray tube in order to increase bearing life, a hollow bearing passage is connected with a source of cooling fluid. The cooling fluid is passed through the hollow bearing passage to maintain an associated bearing assembly below a predetermined temperature.
In accordance with one aspect of the present invention, an x-ray tube assembly comprises an envelope defining an evacuated chamber. The envelope houses a cathode for providing a source of electrons and an anode positioned to be struck by the electrons and generate x-rays. A rotor is operatively connected to the anode for rotating the anode relative to the cathode. The rotor includes a bearing assembly having a hollow bearing shaft. A first air supply is connected to a tube which takes and directs cool air from the first air supply into and through the hollow bearing shaft where bearings of the bearing assembly are cooled.
In accordance with another aspect of the present invention, an x-ray tube includes an anode rotatably connected to a rotor. The rotor has a hollow bearing shaft for receiving cooling oil, air, or gas during operation. Bearings are disposed between the rotor and the bearing shaft. A method of manufacturing such an x-ray tube includes first baking the x-ray tube at a baking temperature. A vacuum is provided for pumping gases and molecules from an interior chamber of the x-ray tube. Concurrently with the baking step, a cooling medium is forced through the hollow bearing shaft to maintain the bearings below the annealing temperature.
In accordance with another aspect of the present invention, a manufacturing assembly for an x-ray tube includes a baking oven. An envelope is disposed within the baking oven. The envelope defines a chamber which houses a cathode for providing a source of electrons. The envelope also houses an anode positioned to be struck by the electrons and generate x-rays. A rotor is operatively connected to the anode for rotating the anode relative to the cathode. The rotor has a bearing assembly having a hollow bearing shaft. A vacuum pump is coupled to a tubular portion of the envelope for removing unwanted molecules and gases from the chamber. A supply pump is provided for supplying a cooling medium through the hollow bearing shaft so that bearing components of the bearing assembly can be cooled.
One advantage of the present invention is that it reduces bearing temperature during exhaust processing and normal operation.
Another advantage of the present invention resides in the ability to bake x-ray tubes at increased temperatures and for longer periods of time.
Another advantage of the present invention resides in the ability to better remove unwanted molecules from a glass envelope of an x-ray tube.
Another advantage of the present invention is that it increases the life of the rotating anode tube.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon a reading and understanding of the following detailed description of the preferred embodiment.